Posidonia

For several years we have taken the students on the Mallorca field-course to the strand-line along the Bay of Pollensa and the dune system near C’an Picafort. Both of these stretches of beach tend to collect odd, fuzzy balls of Neptune’s grass (Posidonia oceanica). Wave action breaks down the dead leaves and rhizomes of Neptune’s grass creating fibres which then become matted into dense spheres. I’ve written a previous blog post about Neptune’s grass on these shores of Mallorca.

Instead, this year we visited a different part of the coast where the material accumulates in sculpted waves along the beach edge. Previously I’ve seen this from the window of the coast, so it was interesting to experience it first hand. It is very soft, prone to collapsing and makes the shore edge difficult to walk on. There must be something different about the coastline here which makes the formation of the fibre balls less likely. Whether in balls or loose, the dried Neptune’s grass adds organic matter to the sand and helps to stabilise the dunes further up the beach.

This bit of beach was at the Finca de Son Real, an example of a traditional land-holding now managed by the Balearic Government as a nature reserve and archaeological site. There is a museum here which gives an insight into the lives of the rural people of Mallorca. Through displays of objects, room reconstructions, audio and projections, the museum explores the site from and from neolithic times into the 20th century including an explanation of how local farmers would have collected dry Neptune’s grass to use as animal bedding.

Now there couldn’t be a finer way to listen to a lecture! These first-year undergraduate students from the University of Manchester are in Mallorca to learn about Mediterranean ecosystems and plant adaptations to the climate. On days like today it’s much easier to explain the challenges of the Mediterranean climate than on days when the rain is falling steadily. They also get to appreciate the view too; this view is out across the Bay of Pollensa where underneath the water there are large beds of seagrass called Posidonia oceanica or Neptune’s grass.

Shadowy underwater meadows

Although underneath the waves, it not a seaweed and is actually a flowering plant. It can produce fruits which float on the ocean, but it also spreads very slowly by creeping rhizomes. A recent study published in the journal Plos One has shown that these clonal meadows of seagrass could be thousands of years old.

A washed up Posidonia oceanica rhizome

Growing at depths of about 1-45m (depending on water clarity) these seagrass beds are very important ecosystems. These meadows trap carbon dioxide and release oxygen in coastal waters through photosynthesis, provide energy at the bottom of the food chain and act as nursery grounds for many fish and invertebrates. The environmental importance of this ecosystem is reflected by its protection as a priority habitat by the legislation of the EU Habitat Directive and by the programs aimed at conserving it.

Sculpted mounds of debris

These meadows also have important effects on the seashore. Debris is washed-up from below the sea and collects on the beaches, sometimes forming great sculpted ‘cliffs’ of plant material. These heaps of dead seagrass leaves and rhizomes are striking, but far more curious are the fibre balls which can also be found on some beaches. Wave action on the beach smashes up and wears away the tissue of the seagrass leaves, leaving only the leaf veins which get tangled up and moulded into these pebble shapes.

Fibre balls in the herbarium collection.Dead leaves and fibre balls on the seashore

Which ever form the material ends up on the beach in, once there it helps to stabilize the shifting sands to build dunes, to retain water and to provide nutrients which allows plants to colonize the strandline. Plants such as this lovely yellow-horned poppy (Glaucium flavum) which sadly isn’t in flower yet.